1. Field of the Invention
This invention relates to measuring rate of rotation. More specifically, the invention is an apparatus and method that develops a DC signal indicative of rate of rotation using a brushless DC motor.
2. Description of the Related Art
Angular rate sensing devices or tachometers are well known in the art. Such devices can be constructed using motors with mechanical brushes, using a brushless DC motor combined with position sensors, or using resolver or encoder developed rate sensors. However, each of these devices has drawbacks.
Tachometers built using motors with mechanical brushes present an ongoing maintenance concern in that the mechanical brushes wear out. Further, mechanical brush-type tachometers are prone to electrical arcing which affects device performance.
Tachometers built using a brushless DC motor use position sensors to commutate the back electromagnetic force (emf) signals of the phases of the motor. However, commutation typically takes place using discrete hall sensors to measure position which causes a relatively large ripple in the rate signal. To control the ripple problem, linear position sensors (e.g., resolvers, etc.) can be used for commutation purposes. However, the fact that a position sensor is still required adds weight, complexity and cost to the overall system. Also, use of any position sensor requires some excitation signal which means that wires must be led to the sensor thereby adding weight and cost. Further, the excitation signal must be produced by some apparatus thereby adding more weight and cost.
A resolver developed rate sensor requires some type of differentiation because a resolver is a position sensor, and position must be differentiated to obtain rate. However, as is known in the art, differentiation circuitry is noisy. A further disadvantage is that an excitation signal is required which requires both the leading of wires to the sensor and the need for apparatus to provide such excitation signal.
An encoder developed rate sensor measures time between discrete pulses of an encoder to provide a position change for a given time change, i.e., a rate. Once again, a drawback associated with this type of device is the need for wires to carry excitation signals to the sensor and the associated hardware needed to generate such excitation signal. Further, encoder developed rate sensors are inherently less accurate (especially at slower rates of rotation) because of the discrete nature of the encoder outputs.